1. Field of the Invention
The present invention relates to heart-monitoring devices, and more particularly to devices which analyze the R waves of an electrocardiogram signal to detect arrhythmias of the heart.
2. History of the Prior Art
Various devices are known for monitoring heart function. Such devices typically function by analyzing signals representative of heart function such as an electrocardiogram (ECG) signal.
Detection devices of this type have found a wide-spread use in conjunction with patients known to have a cardiac problem such as in the case of heart patients in an intensive care unit. Such devices also have many uses outside of the intensive care unit such as in conjuction with persons who are not known to have a cardiac condition as such but who may exhibit suspicious symptoms. For example, a person may have a mild heart attack and not know what it is. However, by the time the person gets to a doctor, the symptoms may have sufficiently subsided so as to make diagnosis difficult. In such situations it would be desirable to be able to monitor the person's heart activity over an extended period of time and outside of the doctor's office. In still other situations, it may be desirable to be able to monitor a person's heart activity over a prolonged period of time to determine whether that person's heart activity is normal and thereby indicate that certain symptoms are psychosomatic or caused by other factors.
A number of different detecting devices are known for monitoring a person's heart activity. Many such devices respond to the reoccurring R wave within the electrocardiogram signal in an attempt to detect improper heart beats or arrhythmias. One of the more common detecting devices of the type which responds to R waves includes what is commonly known as a QRS detector. The QRS detector responds to the beginning of each R wave within an electrocardiogram signal by generating a pulse of fixed width or duration. Further circuitry analyzes the time distance between the generated pulses to make a determination of heart rate or pulse rate. Detecting devices of this type are nothing more than pulse rate monitors and make no attempt to analyze the R waves except in terms of their frequency of occurrence.
Still other detecing devices make some attempt at analyzing various waveform components of the electrocardiogram signal by sensing the zero crossings of such signal. The locations of the zero crossings may then be analyzed such as by comparing their time of occurrence with a reference template representing a desired or normal pattern of zero crossings. The problem with detection devices of this type is that they are highly prone to false triggering in response to noise rather than to valid heart arrhythmias. For example, it is quite easy to respond to and analyze one of the various noise signals commonly generated by the body, thinking it to be the electrocardiogram signal. Still furhter problems stem from the fact that such detection systems typically do not change the reference template or criteria in response to the gradual changes in heart function which occur in everyone. Thus, a reference template which defines normal heart activity for the patient at a given moment may not accurately reflect the same patient's normal heart activity at a later time.